Wavelength modulation photoacoustic spectroscopy: Theoretical description and experimental results

A theoretical description of photoacoustic spectroscopy generated by wavelength modulation of a semiconductor laser source is reported for a Lorentzian absorption line. This model describes the first- and second-harmonic photoacoustic signals produced by a current-modulated semiconductor laser. Combined intensity- and wavelength-modulation is considered with arbitrary phase shift. Experimental results obtained when probing a CO2 absorption line with a 2-μm distributed feedback laser are presented and validate the relevance of the reported model.